1. Field of the Invention
The present invention relates to an optical image retrieval method, and especially relates to an optical input device for retrieving correct images from a transparent medium.
2. Description of Related Arts
A prior art mouse comprises an X-axis encoder and a Y-axis encoder with output logic sequence signals (ex. 11, 10, 00, 01). The mouse is placed on a top surface or other surfaces of a desk and moved in designated directions to place a cursor on a monitor in corresponding positions. The movement of cursor on the monitor made by the mouse adopts a principle that the X-axis and the Y-axis encoders together produce control signals to move the cursor.
Reference is made to FIG. 1A and FIG. 1B, which illustrate cutaway views of prior art optical mice, respectively. When the prior art mouse is moved on a plane, a circuit control unit (not shown) will calculate the distance and direction of the movement of the mouse by the following steps:    1. Light emitted from a light-emitting element 61 is projected to a first reflection surface 621 of a transparent plate 62;    2. The light is then reflected to a second surface 622;    3. The second surface 622 reflects the light onto a contact surface 64 made of a non-transparent interface through an opening of a bottom shell 63, in which an image axis F overlaps with the contact surface 64 when the contact surface 64 is a non-transparent interface;    4. A lens 65 focuses optical image signals of the image axis F onto an image-detecting element 66; and    5. The optical image signals are transferred to the circuit control unit for processing.
An optical axis D of the projected light must be intersected at a point P of the image contacting surface 64 with an image-retrieval optical axis R and an image axis F; therefore, the image-detecting element 66 can retrieve the image on the image axis F.
If the image contacting surface 64 is made of a transparent medium such as glass, the image axis F does not overlap with the image contacting surface 64. The optical axis D of the projected light may intersect with an image axis F1 at a point M under the image contacting surface 64 (the refraction effect is not taken into account here). Meanwhile, the optical axis D and the image-retrieval optical axis R cannot intersect with the image axis F of the image contacting surface 64, as FIG. 1B shows. The optical mouse therefore doesn't work on a transparent medium.
From another point of view, prior art mice must reflect the light emitted from the light-emitting light-emitting element 61 twice to the image contacting surface 64, and this wastes power.